Sulfotransferases (“SULTs”) are a class of enzymes that catalyze sulfonation of compounds that carry either a hydroxyl group or an amino group, and play an important role in metabolism of xenobiotics, drugs and many endogenous compounds. They have a wide distribution in the body and act as major metabolic/detoxification systems (Phase II metabolic enzymes) in regulating homeostasis of the body. They are highly expressed in tissues that are exposed to foreign compound: the lungs and respiratory tract (including the nasal cavity), the gastrointestinal tract, and the liver. In the majority of cases, addition of sulfonate moiety to the structure increases its solubility, because sulfates are fully ionized at physiological pH, and decreases biological activity. Yet these enzymes are also capable of bioactivating procarcinogens to reactive electrophiles. (Falany, 1997; Gamage et al., 2006) The universal sulfonate donor for the sulfonations catalyzed by sulfotransferases is 3′-phospho-adenosyl-5′-phosphosulfate (PAPS). (Robbins and Lippman, 1956)
A variety of drugs and natural products are either substrates or inhibitors of these compounds. For example, several natural compounds that exhibit antioxidant and anticancer activity, such as curcumin from curry powder and epigallocatechins from green tea, are potent inhibitors of phenolic sulfotransferases.
The protective role of these sulfotransferase inhibitors may be better understood if we take into account the involvement of sulfotransferases in the activation of several pro-carcinogenic compounds to fully activated carcinogens. This happens when hydroxyl groups in allylic or benzylic alcohols, or in hydroxylamines formed from aromatic amines, get sulfonated. The resulting sulfates are unstable and readily decompose to inorganic sulfate and highly reactive electrophilic carbonium or nitrenium cations. These species can then attack highly nucleophilic DNA and covalently bind to it, which may lead to mutagenesis and carcinogenesis.
Similarly, several drugs or their metabolites having phenolic hydroxyl groups are also metabolized predominantly through sulfonation. Examples include the cancer drugs raloxifene and tamoxifen:

Sulfotransferases can be divided into two large groups: the cytosolic SULTs and the membrane-associated SULTs, which are localized in the Golgi apparatus. Cytosolic SULTs catalyze the sulfonation of xenobiotics, hormones, monoamine neurotransmitters, and drugs. Orally administered xenobiotics and drugs are to a large extent sulfonated in the intestines and in the liver, and excreted either via urine or via bile. Membrane-associated SULTs catalyze the sulfonation of proteins and carbohydrates for processes such as cellular signaling and modulation of receptor binding, such as glycosaminoglycans, glycoproteins, etc. Recent studies have implicated the SULTs in a number of disease states, including entry of the herpes virus, (Xu ea al., 2006) entry of HIV (Seibert et al. 2002), chronic inflammation (van Zante and Rosen, 2003), and cancer (Suzuki et al., 2003).
Five distinctive cytosolic sulfotransferase gene families have been identified in mammals (SULT1-SULT5), of which only SULT1, SULT2 and SULT4 have been identified in humans. (Hempel, 2005) Ten distinctive members of these 3 families were identified: SULT1: A1, A2, A3, B1, C1, C2, E1; SULT2: A1 and B1; and SULT4A1.
SULT1A sulfotransferases are the most abundant and widespread among the 10 members and they differ based on their metabolic preference for different substrates (phenol sulfotransferase SULT1A1, monoamine sulfotransferase SULT1A3).
The major role of sulfonation in the body is metabolism and detoxification of xeno- and endobiotics via conjugation with sulfonate moiety, which makes the compounds more soluble at physiological pH and easier to excrete. In addition to sulfoconjugation of drugs and xenobiotics having hydroxyl groups (or metabolites hydroxylated by Phase I metabolic enzymes), glucuronidation also may contribute in metabolism and excretion of these compounds. Sulfonation and glucuronidation are complementary conjugation processes and take place in different cellular compartments: sulfonation in the cytosol and glucuronidation in the endoplasmic reticulum. Sulfonation is considered a high affinity, low capacity pathway, whereas glucuronidation is considered to be a low affinity, high capacity pathway. (Burchell and Coughtrie, 1997) In general these two enzyme systems also show differences in tissue localization, with major SULT expression occurring in organs facing high exposure to foreign chemicals (e.g., lungs, intestines) and, at the same time, relatively low UDP-glucuronosyltransferase activity in the same organs. (Pacifici et al., 1998)
In addition to metabolism and excretion, sulfonation also plays a strong regulatory role for several classes of endogenous substrates, including estrone, cholesterol, dopamine, bile acid salts, testosterone, and neuroendocrine peptide cholecystokinin (CCK). For example, in normal human plasma, 99% of total dopamine, 78% of total noradrenaline, and 67% of total adrenaline is present in inactive sulfonated form. (Eisenhofer et al., 1999) Similarly, the level of the inactive sulfonated form of the hydroxysteroid hormone dehydroepiandrosterone (DHEA) in plasma is 100-fold higher than the level of unsulfonated DHEA. (Falany, 1997)
Sulfonation can also increase activity of endogenous molecules, as in the case of the neuroendocrine peptide cholecystokinin (CCK), which exhibits biological activity when in sulfated form. (Vargas et al., 1994)
A variety of xenobiotics are substrates for different SULT1 enzymes in the intestines and liver. Examples are (−)-salbutamol, 7-OH-flavone, paracetamol, and (−)-apomorphine. Some natural products and chemicals are potent inhibitors of SULT1A1 and SULT1A3 activities, including 2,6-dichloro-4-nitrophenol (DCNP), curcumin, and quercitin, among others. (Pacifici, 2005)
The human brain displays a moderate level of SULT1 activity. Sulfonation activity for dopamine (SULT1A3) and for p-nitrophenol (SULT1A1) was measured in 17 brain regions of 6 brains from subjects of age 55-74 years. There were considerable regional differences in both SULT activities, with the values for neocortical regions (frontal, parietal, temporal) significantly higher than for the subcortical regions and cerebellum. SULT1A1 activity observed in the frontal cortex was 4.7 times higher than that observed in the thalamus, and SULT1A3 activity was 4.0 times higher in the frontal cortex than in the thalamus. (Young and al., 1984) The authors also assessed the extent of SULT activity loss as a function of post-lobectomy delay in a study on brain tissue from 5 lobectomy patients. Loss of activity was 20% in the post-surgery brain samples 8 hours after tissue removal. SULT E1has been detected in human brain tissue (Miki, et al., 2002)
The immunohistochemical detection of phenol sulfotransferase-containing neurons in tissue samples from 4 normal brains obtained 6-12 hours after death revealed that the immunostaining was localized to the cytosol of specific neuronal populations in each region analyzed, i.e., the hippocampus, thalamus, striatum, and medulla. (The antibody used was cross-reacting with SULTI1A1 and SULTI1A3.) However, no other areas of cortex (frontal, parietal, or occipital) were analyzed, which prevents us from making any significant correlations with the study of Young et al.
R(−)-apomorphine, a drug used for treatment of Parkinson's disease has been found to be sulfonated by the brain SULT1A enzymes. As in the case of liver and intestinal sulfonation of this drug, it can be blocked with quercitin with IC50 value of 16±2.3 nM. (Vietri et al., 2002b).
In addition to the SULT1 family, the SULT4 family has been discovered and was found to be localized in brain tissues only. (Liyou et al., 2003) Although the authors were unable to identify the substrates for this enzyme, we can conclude, based on very high inter-species preservation of the enzyme structure, that it is involved in an important process.
The Role of Sulfotransferases in Disease
The interaction between sulfotransferases and carcinogens, i.e. activation and inactivation of carcinogens by sulfotransferases, has been extensively studied. (Glatt, 2005). A majority of the pro-carcinogens are aromatic compounds, which can be easily functionalized in such manner that they contain either a benzylic or allylic alcohol or an aromatic hydroxylamine structural unit. The hydroxy group is readily sulfonated by the sulfotransferases, yielding activated sulfuric acid esters of benzylic and allylic alcohols, and aromatic hydroxylamines, which readily lose the sulfate moiety to form resonance-stabilized carbonium or nitrenium ions. These reactive electrophilic species then react with nucleophilic sites on DNA, leading to mutagenicity and carcinogenicity.
The polymorphism of the sulfotransferase SULT1A1 gene leads to Arg-to-His substitution at the codon 213, which leads to lower activity and thermal stability of the enzyme. Several studies have tried to link the decreased activity of SULT1A1 to higher risk for several types of cancers, including gastric cancers among males who drink and smoke (Boccia et al. 2005), head and neck cancer amongst older people who are alcohol and low fruit consumers (Boccia et al., 2006), esophageal cancer in men (Wu et al., 2003), lung cancer in current smokers and current heavy smokers (Wang et al., 2001), and breast cancer (Shatalova et al., 2005).
The SULT1A1 and SULT1A3 sulfotransferases were reported to have increased activity in the thyroid glands of autoimmune thyroid disease patients, and SULT1A1 activity was elevated in nodular goiter patients when compared to healthy controls. (Ebmeier and Anderson, 2004)
Neurodegenerative diseases are another class of diseases with sulfotransferase involvement. A study of SULT1A3 activity performed on brain tissue samples from 7 Parkinson's disease patients and 8 controls suggests that SULT1A3 activity was decreased in all neocortical areas of PD patients (20-39% of controls) when compared with controls, but was increased in the caudate nucleus area with pathology in PD (174-203% of controls). (Baran and Jellinger, 1992)
Although there are no direct reports on the fate of sulfotransferases in Alzheimer's disease (AD), the most common neurodegenerative disease, there is a wealth of evidence that the neuropathological fibrillar deposits found in AD, namely β-amyloid plaques and neurofibrillary tangles, are all associated with heparan sulfate proteoglycans (van Horssen et al., 2003; Verbeek et al., 1999), which may be aiding in fibrillogenesis and also have some anti-protease activity which would protect the amyloid aggregates from degradation the same way as heparan sulfate proteoglycans protect basic fibroblast growth factor from proteolysis when bound to it. (Saksela et al., 1988)
In a way analogous to Parkinson's disease, where the highest SULT1A3 activity is observed in areas having the strongest neurodegeneration in the dopaminergic brain system, we predict that the significant changes of activity and/or expression of SULT1 enzymes in Alzheimer's disease will reflect the global cortical and subcortical distribution of neurodegenerative processes (i.e., neuronal loss, synaptic loss, and formation of neurofibrillary tangles and β-amyloid plaques.
Alzheimer's Disease
Alzheimer's disease is a progressive neurodegenerative disease that affects approximately 20-40% of the population over 80 years of age, the fastest growing age group in the United States and other post-industrial countries. Common features in the brains of patients with Alzheimer's disease include extensive loss of neurons from the vulnerable neuronal population and the presence of neuropathological deposits, including β-amyloid senile plaques (SP) and neurofibrillary tangles (NFT)s. As the disease worsens, the deposits spread throughout the brain in a predictable pattern starting from the medial temporal lobe and progressing gradually to the rest of the cortex. (Braak and Braak, 1991) Related pathologies are seen in other forms of dementia (e.g. Frontotemporal Dementia) and in Down Syndrome.
Since the initial deposits occur much earlier than the symptoms of the disease, early in vivo imaging of these deposits has tremendous diagnostic value. A method using “[18F]FDDNP” (2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malononitrile), a [F-18]fluorine labeled probe, has been developed for in vivo detection of neuropathological deposits in Alzheimer's disease with positron emission tomography (PET). It has been used to determine in vivo levels of the pathological deposits present in brains of Alzheimer's disease patients, and consistently shows higher levels of [18F]FDDNP binding in the brain areas with known pathology when compared with the levels determined in the same areas in the brains of cognitively normal, age-matched subjects (Shoghi-Jadid et al., 2002; Kepe et al., 2004). In vitro results demonstrate the capacity of [18F]FDDNP to bind to both major types of neuropathological deposits found in Alzheimer's disease: extracellular β-amyloid senile plaques (SP) and intracellular neurofibrillary tangles (NFT). (Agdeppa et al., 2003a) This binding can be selective blocked by naproxen, a non-steroidal inflammatory drug. (Agdeppa et al., 2003b)
Two other types of [11C] carbon-radiolabeled compounds suitable for PET have been developed and are reported to have specific binding to β-amyloid fibrillary aggregates and used for brain PET imaging in Alzheimer's disease. The first is a benzothiazole derivative: 2-[(4′-[11C]methylamino)phenyl]-6-hydroxybenzothiazole (“[11C]6-OH—BTA-1” or “PIB”), which has been used to study Alzheimer's disease patients and controls with PET (Klunk et al., 2004). The second is a stilbene derivative: 4-([C-11]methylamino)-4′-hydroxystilbene, which has been developed for PET studies and used in a small study with 3 Alzheimer's disease patients and 3 controls. (Verhoef et al., 2004). The results are comparable to the results obtained with [11C]6-OH—BTA in the same subjects. As both [C-11]6-OH—BTA and 4-([11C]methylamino)-4′-hydroxystilbene are phenolic compounds and therefore sulfotransferase substrates, it is reasonable to assume that at least part of the observed signal results from the sulfonation of both probes in the brain and retention of the resulting sulfates. Indeed, [11C]6-OH—BTA has been shown to be sulfonated in the rat brain in vivo. (Mathis et al., 2004)
Although it is recognized that sulfotransferases play an important role in health and disease, no method currently exists for measuring sulfotransferases in vivo, and much remains to be learned about their distribution in the body and their role in the evolution of diseases such as Alzheimer's Disease, cancer, and lung disease related to smoking. An urgent need exists for a method of monitoring the distribution and activity of SULTs in vivo, and for assessing the effect of therapeutic interventions aimed at this class of enzymes.